A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The substrate support tables used in lithography have a support surface with protrusions that extend substantially perpendicularly from the surface. In operation, the backside of the substrate is supported on the protrusions, at a small distance from the support surface of the substrate table, in a position substantially perpendicular to the direction of propagation of the projection beam. Thus, the tops of the protrusions, rather than the support surface of the wafer table, define an effective support surface for the substrate.
In order to avoid overlay errors during projection of patterned beam of radiation on a substrate, it is desirable that the substrate top surface be flat. Unevenness of the supporting surface of the substrate support may lead to an uneven top surface of the substrate supported on the support surface. Therefore, it is desirable to avoid unevenness in the substrate support.
Unevenness of the supporting surface can be caused by dissimilarity between the heights of material that makes up the protrusion itself. This is typically the case when a new substrate support table has been manufactured. Possibly uneven wear may also lead to unevenness. In a known embodiment, the substrate support table contains a chuck on which the table with the protrusions is supported. In an alternative embodiment the chuck and substrate support table may be integrated in a single unit. Unevenness may be the result of differences between the heights of the protrusions, or in the backside of the table or in the chuck. Therefore these elements are carefully made level. Nevertheless it has been found that unevenness may also result when the chuck and the support table (and any other elements) are assembled or installed. Similar problems may be encountered with support tables for other articles that have to be supported in a well-defined plane across the beam path, such as reflective patterning devices or transmission patterning devices
US 2005/0061995 A1, the contents of which is herein incorporated by reference in its entirety, provides a lithographic projection apparatus including a detector to detect height deviations of the protrusions that affect a surface flatness of the article, a height adjustment device arranged to independently modify a height of the protrusion material of individual protrusions when the support table is operable in the apparatus, and a controller coupled between the detector and the height adjustment device and arranged to control the height adjustment device to adjust the height of the protrusions corresponding to the detected height deviations of the protrusions that affect the surface flatness of the article.
An in situ height adjustment device is used to alter the height of the material that at least the top of individual protrusions are integrally made of, when the support table is at an operable position in the lithographic projection apparatus. By “operable,” it is meant that the support table may be moved to a pattern projection position in the apparatus from the operable position without movements that are more disruptive to the support table assembly than during normal use. “Integrally made” refers to material that is used to manufacture the support table or coatings or other material layers on the protrusions, but not to accidental foreign material such as pollution. By adjusting the height of the protrusions in the assembled support table in the lithographic apparatus, at such an operable position, a reliable local and global height adjustment may be realized.
A detector determines which of the protrusions have a height deviation and a control unit controls the height adjustment device, for example, to remove a part of the material of selected protrusions with excess height, but not from other protrusions that do not have an excess height, or an excess height below a threshold.